It is well known in the art that semiconductor material such as doped silicon possess piezoresistive characteristics. This simply means that the electrical resistance of the semiconductor material changes when the material is subjected to strains such as bending. The change in resistance, and hence the strain applied to the semiconductor material can be measured accurately. This material and its capabilities can be used as a piezoresistive sensor.
One method for making such a device is to simply take a piece of doped silicon and bond it to a strain receiving member by an adhesive. The strain-receiving member is typically a flexible metal sheet, bellows or diaphragm. The opposed side of strain receiving member is exposed to the media that is being measured. Bending of the membrane induces strain; hence resistance change, on the gauges. The major drawback to this glued sensor technology is its susceptibility to output drift. As the sensor ages the bond between the semiconductor material and strain-receiving member also changes.
U.S. Pat. No. 5,518,951 describes a sensor formed by applying two or more insulative silicon layers directly to a strain-receiving member such as a metal sheet, bellows or diaphragm. A layer of doped silicon is applied to the insulative layers. Metal contacts for connection between the resistive measuring device and the yet to be formed piezoresistive sensors are then formed at selected predetermined locations. The nonconductive doped layer is then selectively activated in specific locations between the metal contacts to form the resistive sensors. A laser of suitable wavelength is used to activate the doping agent in the layer into activation and conductivity. This causes the layer between the metal contacts to heat, anneal and recrystalize thereby causing the doping atoms into conductivity and form the piezoresistive sensors between the metal contacts.
The sensor fabrication technique of the '951 patent leaves much room for improvement. As the resistor is formed after placement of metal contacts, it can only be formed between the adjacent edges of the pads. As the activation only occurs where the laser can reach, the area of contact between the resistor and the metal pads is often no more than a thin contact line formed between the edge of pads and the resistor. Even placing the laser at an angle smaller than 90.degree. to the surface of the doped layer has failed to provide any incremental area of contact between the pad and the resistor. Any thermal distortion can disrupt this thin connection causing the resistor to fail.
Accordingly, there is a need for a more robust design capable of withstanding thermal distortion and which can be formed in a method for high volume fabrication.